Hackish opposite of the techspeak term top-down design. It has been received wisdom
in most programming cultures that it is best to design from higher levels
of abstraction down to lower, specifying sequences of action in increasing
detail until you get to actual code. Hackers often find (especially in
exploratory designs that cannot be closely specified in advance) that it
works best to build things in the opposite order, by
writing and testing a clean set of primitive operations and then knitting
them together. Naively applied, this leads to hacked-together bottom-up
implementations; a more sophisticated response is middle-out implementation, in which scratch
code within primitives at the mid-level of the system is gradually replaced
with a more polished version of the lowest level at the same time the
structure above the midlevel is being built.